Telephone and other telecommunications circuits operate in an environment where they occasionally may become subjected to an unexpectedly high current that could be injurious to the user or to equipment, unless electrical protection is provided. Fuses and circuit breakers are the usual methods used to provide such protection. The fuse or circuit breaker is installed in the circuit in order to interrupt the circuit before a harmful current reaches protected users and equipment.
Uninterrupted telephone and telecommunications service is an important feature that phone customers have come to expect and demand. While a telecommunications customer will accept a service interruption for safety reasons, an interruption for nearly any other reason is unacceptable. This includes any interruption that may be required for service and maintenance purposes. In order to provide this quality of service, certain levels of redundancy are required in order to provide protection circuitry for telecommunications equipment.
Redundant protection circuitry is required because fuses frequently must be replaced during the course of providing normal service and maintenance while operating telecommunications systems. For example, fuses frequently must be replaced due to changing customer requirements. When customers change their requirements, the usual result is that the circuit load also changes. If there is a change in the circuit load, the fuse must be replaced with one having the correct amperage rating for the new load being carried. The only way to replace the fuse with one of the correct amperage, without causing a service interruption, is to provide a backup fuse that provides circuit protection while the primary fuse is being replaced.
A telecommunication or telephone circuit is usually routed through a central office or centralized maintenance facility where the fuse protection circuitry is located. The prevailing practice is to provide each telecommunication or telephone circuit with two permanently mounted fuses and a switch. One of the fuses is a primary fuse to provide circuit protection during normal operations, while the other is a backup fuse, the sole purpose of which is to provided circuit protection while the primary fuse is being replaced.
In order to change the primary fuse, the switch is manually activated to reroute the telephone circuit to flow through the backup fuse. Service is not interrupted because the switch is activated in a "make-before-break" application. After the primary fuse has been replaced, the switch is again engaged in a "make-before-break" application to restore the telephone circuit to flow through the primary fuse.
There is an additional cost in providing uninterrupted service of the type described. Such cost is found primarily in the number of components, such as duplicate fuses, required to provide continuous protection and in larger facilities necessary to accommodate the cabinets and panels where the fuses and switches are housed or mounted. Such additional costs are significant. If a single fuse protection system could be used to protect each telephone circuit, while still permitting service and maintenance personnel to change a fuse without causing a service interruption, the number of components would be reduced and less space would be required to house the system. If fewer components could be used and less space was required to house the circuitry, the cost of providing a protection system would be reduced. Such a cost reduction would eventually redound to the customer's benefit.
Accordingly, what is needed in the art is a single fuse method to protect a telephone circuit that will permit the replacement of a primary fuse without causing a service interruption, while simultaneously maintaining necessary protection to users and equipment.